This invention relates in general to X-radiation protection systems in a television receiver and in particular to an improved high voltage shutdown circuit for automatically controlling the shutdown voltage of a television receiver.
A cathode ray tube (CRT) in a typical color television receiver requires several operating potentials the highest of which is the accelerating potential of approximately 25 to 30 kilovolts. This voltage, which is generally referred to as the high voltage, is applied to an anode within the CRT to accelerate the emitted electrons to an energy level sufficient to cause light emission upon impact with the phosphor-coated faceplate of the CRT and illumination of the viewing screen. Two operating parameters, the electron accelerating potential and the electron beam current, are particularly important in establishing CRT performance levels. For example, CRT viewing screen brightness is determined primarily by electron beam current intensity. In general, optimum television receiver performance requires operating at the highest possible electron accelerating potential and electron beam current.
One of the most important limitations placed upon these operating parameters is related to safety. Energetic electrons striking the faceplate of the CRT produce X-radiation. If the television receiver is operated at an electron accelerating voltage higher than that for which it was designed, the rate of X-radiation emission may exceed a predetermined maximum safe level. Operation of the television receiver at a voltage higher than that for which it was designed may be due to any one of a variety of factors such as a transient high voltage surge, a faulty high voltage regulating component or an improper receiver voltage setting. Whatever the source, this over-voltage situation is made even more dangerous because of the difficulty in detecting it. Indeed, even with a dangerous increase in operating voltage the television receiver may continue to operate satisfactorily or even with an improvement in performance due to enhanced video presentation brightness. Increased interest in consumer protection combined with increasing television receiver operating voltages have, therefore, prompted much work in the area of further assuring safe CRT operation at acceptable operating voltage levels.
In general, the high voltage, or flyback, transformer is energized by the horizontal drive circuit. The horizontal drive circuit, in turn, is driven by a series of synchronization input pulses controlled by a pulse width modulator. The output of the horizontal drive circuit energizes the primary of the high voltage sweep transformer which in turn drives the tertiary winding of the high voltage sweep transformer. The tertiary winding provides approximately 30 kilovolts DC to the CRT's ultor, or anode, for tube turn-on. In addition, the tertiary winding provides current to the Automatic Brightness Limiter (ABL) which provides current to the CRT's cathode and various control grids. The conventional high voltage shutdown system monitors the voltage level applied to the high voltage anode and provides a shutdown signal to the horizontal drive circuit through the pulse width modulator when the anode voltage level exceeds a predetermined value. The feedback signal provided to the pulse width modulator is proportional to the absolute value of the detected high voltage level. This results in the establishment of a high voltage limit independent of electron beam current even though both of these operating parameters, in combination, establish the level of X-radiation emission.
One approach to solving the problem of X-radiation in the CRT of a television receiver is disclosed in U.S. Pat. No. 3,644,669. The system described therein utilizes a current transformer to sense the pulsating DC current in the high voltage lead of the CRT to generate a control voltage corresponding to the average beam current intensity. This control voltage operates a threshold circuit which limits the drive to the CRT when the average beam intensity exceeds a predetermined level. While providing an effective means for limiting CRT beam intensity, this approach, because it derives the feedback control signal directly from the high voltage source to the CRT, is subject to high voltage failure modes thus requiring the use of more expensive components for operating in the high voltage environment.
Another approach to solving the problem of overvoltage operation in a television receiver is disclosed in U.S. Pat. No. 4,047,078. The invention described therein, which was designed primarily to provide a high voltage shutdown circuit possessing greater immunity to erroneous triggering, involves an overvoltage detection system which senses variations in the amplitude and duration of the retrace portion of the horizontal scansion signal. Thus, this system only indirectly senses either electron accelerating potential or electron beam current intensity in correcting for an overvoltage situation which could lead to the emission of hazardous X-radiation. A more desirable approach would be to more directly sense CRT voltage in generating the approach feedback control signal.
Still another approach to providing overvoltage protection in the CRT of a television receiver is disclosed in U.S. Pat. No. 3,692,933. The overvoltage protection circuit disclosed therein samples an output of the high voltage transformer to determine the high voltage level of the television receiver. The protection circuit then generates a control signal which is coupled to an input to the automatic gain control circuit of the television receiver which, in turn, produces a signal for limiting the receiver's gain in proportion to the strength of a synchronizing signal component of a broadcast television signal selected by the tuner and applied to the automatic gain control input. The output signal of the overvoltage protection circuit to the automatic gain control circuit overrides the incoming synchronizing signal when the high voltage output rises above a predetermined level with the result that all inputs to the television receiver's CRT are terminated. Thus, this approach also represents a more indirect means for controlling the voltage applied to the CRT. Rather than applying voltage corrections directly to the CRT, this system adds to the television receiver's complexity by interfacing with the CRT through the automatic gain control circuit of the television receiver. Other, less pertinent, attempts at providing overvoltage protection in a television receiver are disclosed in U.S. Pat. Nos. 3,649,901 and 3,715,464.
Regardless of the approach taken, the prior art has not been able to adequately deal with the unique relationship between television receiver high voltage and beam current intensity in providing safe operation at optimum brightness levels. An inverse relationship exists between voltage level and beam current intensity as they relate, in combination, to the assurance that predetermined maximum levels of X-radiation emission are not exceeded. For example, as the voltage increases the beam current must decrease in order to avoid the emission of X-radiation. This relationship between high voltage and beam current intensity is generally provided by a CRT manufacturer for a particular tube in the form of an "Isodose" curve. Voltagebeam current combinations above the "Isodose" curve produce X-radiation while voltage-beam current combinations below the curve provide for safe television receiver operation. While the prior art fails to take into account the relationship between these two CRT parameters as they relate to safe television receiver operation, the present invention is designed to permit television receiver operation at higher electron accelerating potentials and increased beam current intensities while maintaining safe operation at optimum brightness levels.